In recent years, the liquid crystal display elements are expected not only in the conventional application to personal computer monitors but also in application to ordinary color televisions. The color reproduction range of the color liquid crystal display elements is determined by colors of light emitted from the red, green and blue pixels and, where chromaticity points of the respective color pixels in the CIE XYZ colorimetric system are represented by (xR,yR), (xG,yG) and (xB,yB), the color reproduction range is defined by an area of a triangle surrounded by these three points on an x–y chromaticity diagram. Namely, the larger the area of this triangle, the more vivid color image the display elements reproduce. The area of this triangle is normally expressed using a ratio of the area of the triangle to an area of a reference triangle formed by three points of the three primary colors, red (0.67, 0.33), green (0.21, 0.71) and blue (0.14, 0.08), in the standard system defined by U.S. National Television System Committee (NTSC) (in unit of %, which will be referred to hereinafter as “NTSC percentage”). The ordinary notebook computers have the values of approximately 40 to 50%, the desktop computer monitors the values of 50 to 60%, and the existing liquid crystal TVs the values of approximately 70%.
A color liquid crystal display device utilizing such color liquid crystal display elements is mainly composed of light shutters utilizing a liquid crystal, a color filter having red, green and blue pixels, and a backlight for transmission illumination, and the colors of light emitted from the red, green and blue pixels are determined by the emission wavelengths of the backlight and the spectral curve of the color filter.
The backlight generally used is one using as a light source a cold-cathode tube with emission wavelengths in the red, green and blue wavelength regions and using a light guide plate for converting light emitted from this cold-cathode tube, into white area light source. Among illuminants of the cold-cathode tube, the red illuminant is made from an Y2O3:Eu type phosphor, the green illuminant from a LaPO4:Ce,Tb type phosphor, and the blue illuminant from a BaMgAl10O17:Eu type phosphor or an Sr10(PO4)6Cl2:Eu type phosphor, as a typical example. A fluorescent lamp is used as a light source for the backlight in structure in which the electrodes are mounted in a sealed package provided with a phosphor film formed from a mixture of these phosphors at an appropriated compounding ratio in consideration of the white balance and in which the interior is filled with a rare gas.
Another backlight uses a substrate provided with a phosphor layer, and a cathode tube or an LED to emit ultraviolet, blue or deep blue light, and is configured to excite the phosphors by light therefrom and use their luminescence as a white area light source.
In the color liquid crystal display elements, the color filter extracts only wavelengths in necessary regions from the emission distribution of the backlight as described above, to provide the red, green and blue pixels.
Methods for production of this color filter proposed heretofore include such methods as dyeing, pigment dispersion, electrodeposition, printing, and so on. The colorants for coloring used to be dyes, but are now pigments in terms of reliability and durability as liquid crystal display elements. Accordingly, the pigment dispersion is most commonly used as a method for production of the color filter at present in terms of productivity and performance. In general, in use of an identical colorant, the NTSC percentage and brightness are in a trade-off relation and are appropriately used according to applications.
Incidentally, there are recently increasing demands for color liquid crystal display elements capable of expressing more vivid color images, while further expanding the color reproducibility of the liquid crystal display elements. Specifically, there are needs for displays with high chromatic purity of NTSC percentage of at least 80%.
However, the backlight using the aforementioned phosphors shows emissions in the other wavelength regions than red, green and blue, as sub-emissions as shown in FIG. 2, and these can be the cause of degrading the chromatic purity. Namely, these sub-emissions impede the expansion of the color reproduction range of the liquid crystal display elements.
A large amount of pigments are necessary for adjustment on the color filter side to adequately suppress the sub-emissions so as to improve the chromatic purity. However, the pigments originally have the characteristic of the spectral curve being not sharp, and thus the use of the large amount of pigments posed the problem that absorption also increased in the principal emission regions and the resultant image became dark as a whole. There were also the following problems: an increase of the pigment concentration in each pixel of the color filter degraded the performance as a photolithographic material, e.g., it resulted in increase of development time; it became difficult to control the pattern shape; the yield decreased; and so on. Furthermore, an increase of thickness of the color filter tended to cause trouble more readily in production steps of a liquid crystal panel and, in turn, led to an increase of production cost of the liquid crystal display device.
An improvement proposed to solve the above problems is a method of producing the color filter as a thin film at a high pigment concentration, by providing no resist performance to the color layer itself and effecting etching using a positive or negative type resist formed on the color layer. However, this method is not preferable because of its complex steps and increase of production cost as a result.
Furthermore, it is virtually impossible to achieve reproduction with ultrahigh chromatic purity of NTSC percentage of at least 95%, by the conventional backlights using the aforementioned cold-cathode tube. The main reason for it is that the conventional backlights have a principal emission peak as green emission wavelengths in a range of from 540 to 550 nm, as shown in FIG. 2. Namely, the green among the three primary colors in the NTSC system has the chromaticity coordinates of (0.21, 0.71), and the principal emission peak of from 540 to 550 nm is too yellowish to achieve the chromaticity coordinates.
In order to achieve the NTSC percentage of at least 80%, it is essential to improve the backlight, but it is not sufficient. Namely, it is also necessary to improve the color filter for spectrally separating the light from the backlight into the colors of the respective pixels, in conjunction with the improvement of the backlight. As an example, since the ordinary phosphors for green have the principal emission peak in the region of from 540 to 550 nm, a colorant for the green pixels of the color filter is adjusted so as to achieve as high a transmittance as possible in the wavelength region and efficiently absorb emissions from the blue phosphor and red phosphor in consideration of light utilization efficiency. However, for example, if there is a change in the green emission wavelengths of the backlight, the balance will be lost between those in the green pixels of the same color filter. On the other hand, as to the red pixels and the blue pixels, there can occur a state in which an emission appears in a wavelength region in which the emission was weak before and in which the color filter was not required to strongly absorb the emission from the backlight, and it thus becomes necessary to adjust the colorants in connection therewith.
Under such circumstances, for example, JP-A-9-97017 describes that a phosphor having no emission peak in a region of from 470 to 510 nm is used for the light source of the backlight and that the emission spectrum of the phosphor is different from those of the ordinary green phosphors, as is the case in the present invention, but it does not take into account a combination with an appropriate color filter suitable for the light source and thus fails to achieve the ultrahigh chromatic purity of NTSC percentage of at least 80%.
For these reasons, the simple improvement in the emission wavelengths of the backlight cannot achieve the ultrahigh chromatic purity of NTSC percentage of at least 80% or even at least 90%, while the conventional color filter is used as it is.
On the other hand, the problem of sub-emission is also prominent about the red pixels. Namely, the conventional red phosphors have a red emission peak near the wavelength of 610 nm, and the green phosphors have the sub-emission near the wavelengths of from 585 to 590 nm. It is, therefore, necessary to make a definite contrast of transmittance in a small interval of 20 nm between the wavelengths of 590 and 610 nm. It is, however, impossible to achieve the adequate contrast in this wavelength region by the presently, industrially available colorants such as pigments and dyes. As a result, in order to obtain the red pixels with high chromatic purity, the pigment had to be used in large quantity with an inevitable sacrifice of brightness.
Furthermore, the chromaticities of the red pixels normally used at present are of the most reddish (less yellowish) type and are near the chromaticity (0.65, 0.33) in the CIE XYZ calorimetric system. However, the red pixels of much stronger red are effective in expanding the color reproduction range. However, a further shift of the red pixels toward red will result in darkening images. Namely, it is the present status as to the red pixels that a compromise must be made at some point of balance between brightness and the color reproduction range.
Furthermore, with the backlight made from the conventionally used red phosphor having the emission peak near 610 nm, the purity of red was not sufficient and it was difficult to reproduce adequately deep red images.
The first invention of the present application has been accomplished under such circumstances and it is, therefore, an object of the invention to provide a color liquid crystal display device capable of achieving reproduction of a deeper green image by green pixels with high chromatic purity, without sacrificing the brightness of the image, and thereby realizing a vivid color image indicating the high chromatic purity of NTSC percentage of at least 80% or even at least 90%.
It is also an object of the second invention of the present application to provide a color liquid crystal display device capable of achieving reproduction of a deeper red image by red pixels with high chromatic purity, without sacrificing the brightness of the image, and thereby realizing a vivid color image indicating the high chromatic purity of NTSC percentage of at least 70% or even at least 80%.